1. Field of the Invention
The present invention relates to rotary electric machines, particularly to rotary electric machines having an armature winding of a series winding and/or a parallel winding.
2. Description of Related Art
A typical rotary electric machine has, along the inner periphery region of a stator core, a number of axially extending slots, in which an armature winding is contained. Generally, large-capacity rotary electric machines have two armature bar layers in each slot, and two different armature bar layers respectively in different slots are connected to each other to form a one-turn winding. And, pluralities of one-turn windings are serially connected to achieve a high output voltage.
A different rotary electric machine requires a different optimum design and manufacturing method according to electrical specification. The capacity of a rotary electric machine is predominantly determined by the product of output voltage and current. Here, the output voltage and current are respectively constrained by the dielectric strength of a winding conductor used and temperature rise therein, and there is thus needed a design balance between output voltage and current. An approach to reduce the output voltage of a rotary electric machine to below the dielectric withstand voltage of its winding conductor is to configure the armature winding into a parallel circuit. In the case of two-pole rotary electric machines, there are known other than a series winding, for example, an armature winding having two parallel branches and armature winding having three parallel branches (e.g., “Operation and Maintenance of Large Turbo-Generators”, IEEE PRESS, 2004, p. 62; JP-A-2000-50549; and JP-A-2001-309597).
However, changing the output voltage specification of a rotary electric machine, even if the capacity thereof is the same, often involves a design change in the number of the stator slots, stator axial length, etc., resulting in the need for a totally new design and manufacturing method of the rotary electric machine. This increases design and manufacturing lead times, leading to cost increases. Therefore, it is desirable to be able to change the output voltage of a rotary electric machine having fixed external dimensions with minimum design change.
The output voltage of a rotary electric machine such as a generator is inversely proportion to the number of the parallel circuit branches of the armature winding. Specifically, assuming that a relative value for the output voltage of a rotary electric machine having a series armature winding is 1, then relative values for the output voltages of rotary electric machines having two and three parallel armature winding branches are 0.5 and 0.33, respectively. Therefore, a design change of the output voltage of a rotary electric machine can be made by changing the number of the parallel circuit branches of the armature winding. This, however, excessively increases the output current by a factor of 1.5 to 3. Therefore, when the output voltage specification of a rotary electric machine having fixed external dimensions is changed by merely changing the number of the parallel armature winding branches, there is required a design that must allow sufficient margin for dielectric strength and temperature increase in the armature winding, incurring unintended cost increase.
If the output voltage specification of a rotary electric machine having fixed external dimensions can be changed in the range of ratio less than 1.5, this is advantageous for reducing cost of the rotary electric machine. However, in conventional rotary electric machine armature winding configurations, the range of such design change cannot be reduced. Further, conventional rotary electric machine armature winding configurations have a design problem in that merely changing the combination of the series and parallel windings cannot offer sufficient design freedom.